In the fabrication of electronic devices, it is frequently necessary to provide a patterned layer of a metallic conductor which functions, e.g. as an interconnection among a plurality of devices on a substrate, as a gate electrode for a field effect transistor (FET), and the like. One method of forming a patterned layer of metallization is to coat the entire substrate and then wet or dry etch away a portion of the coating, leaving only the desired pattern. This method suffers from a number of art-recognized disadvantages. An alternative is the use of lift-off technique.
Conventional lift-off structures comprise a lift-off layer and at least one overlying resist layer. Upon irradiation and development, the lift-off layer must undercut the resist layer, ideally to a predictable degree. The resultant profile of the structure must be such that a layer of metallization deposited thereover will break off at the openings in the resist structure and deposit in the bottom thereof on the substrate with sufficient clearance to the sidewalls to enable a solvent to penetrate to the substrate and cause the lift-off layer to separate. It is difficult to produce the resist profile necessary for lift-off with the reproducibility essential for commercial use. There have been a number of varied approaches to achieving this object. None can be categorized as being eminently successful.
The problem of developing a successful lift-off technique is heightened by the demand to reduce the width of isolated metal structures formed thereby to one micrometer and, preferably, less than one micrometer. Submicrometer dimensions are desirable to increase the number of devices in a given area of substrate surface and, in the instance of an FET, increase the operating efficiency of the device. It is recognized by those of ordinary skill in the art that decreasing the width of the gate electrode of an FET permits an increase in the maximum operating frequency of the device.
The formation of patterns of metallization of sub-micrometer dimension by combining lift-off with the oblique deposition of a metal layer is disclosed in Fabian, U.S. Pat. No. 4,525,919, issued July 2, 1985 and Kraus, U.S. Pat. No. 4,529,686, issued July 16, 1985. The processes disclosed therein, however, are complex and involve multiple layers of various materials to form a resist structure suitable for lift-off. In accordance with this invention, there is provided a process for deposition of sub-micrometer metallization which is advantageous in that it is comparatively simple and requires only a single layer of resist material.